A PWM (Pulse Width Modulation) control is one example of a control method for driving a three-phase AC electric motor. The PWM control controls an electrical power converter which converts DC (Direct Current) voltage (DC electrical power) to AC voltage (AC electrical power) on the basis of a magnitude relation between phase voltage command signal, which is set from a point of view of making phase current which is supplied to the three-phase AC electric motor to coincide with a desired value, and a carrier signal having predetermined frequency. Incidentally, the PWM control may be used to control an electrical power converter which converts the AC voltage to the DC voltage (see a Patent Literature 1).
By the way, a smoothing condenser for suppressing variation of the AC voltage that is inputted to the electrical power converter or that is outputted from the electrical power converter is often connected electrically in parallel to the electrical power converter. Recently, the smoothing condenser is often downsized by reducing capacity of the smoothing condenser. However, if the capacity of the smoothing condenser is reduced, there is a possibility that ripple (what we call, a pulsation component) of voltage between terminals of the smoothing condenser becomes relatively large. Thus, the Patent Literature 1 discloses a technology that uses third harmonic signal for suppressing (reducing) this ripple of the voltage between the terminals of the smoothing condenser. Specifically, the Patent Literature 1 discloses the technology that controls a switching element of the electrical power converter so that current waveform of input current from an AC electrical power source coincides with combined wave of the third harmonic and sine wave whose frequency is same as that of the AC electrical power.